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Yellowstone National Park Report

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We assessed aspen stand conditions in 2012 in 87 stands randomly located across the northern winter ungulate range of Yellowstone National Park (YNP), and compared these data to baseline conditions measured in 1997-98 shortly after wolves were reintroduced. In 1997-98, browsing rates (the percentage of leaders browsed annually) in aspen stands were consistently very high, averaging 88% of stems browsed; only 1% of young aspen in sample plots were taller than 100 cm and none were taller than 200 cm, the height at which aspen begin to escape from browsing by elk. Using the same methods in 2012, 17 years after wolf reintroduction, browsing rates were much lower averaging 44%, 34% of sampled young aspen were taller than 100 cm, and 5% taller than 200 cm. Mean heights of young aspen in 2012 were inversely correlated with browsing intensity (R2=0.64, p=<0.001), but heights were not associated with current annual growth in height (an index of site productivity; R2= 0.02, p=0.2). Some stands were still heavily browsed in 2012 with wide spatial variation across the range and between stands, but on average, browsing declined after 2003 followed by a relatively rapid increase in height of the tallest saplings. The greatest change was on the eastern side of the northern ungulate winter range, corresponding with recent changes in elk population distribution. Recent growth of young aspen into tall saplings will likely result in the regeneration of overstory trees and the persistence of aspen stands into the future, though aspen recovery will take many years and some stands may continue to decline. These results support the hypothesis that a trophic cascade following the return of wolves to Yellowstone has begun to reverse the decades-long trend of aspen decline on the northern range.